Patterns of Anaerobic Glycerol Utilization in Redox Reactions

Abstract For the first time, this study describes a bacterial redox process of anaerobic degradation that simultaneously involves two key organic osmoprotectants: glycerol as an electron donor and betaine as an electron acceptor. Its stoichiometry was determined, and the biochemical characteristics that enable the combined utilization of these two compounds were identified. The quantitative ratio between glycerol and betaine depended on the type of metabolism and was 1 : 1 in organisms capable of glycerol fermentation and 1 : 2 in organisms that use it only in redox reactions. Genome annotations were used to infer glycerol degradation pathways that account for metabolic differences among microorganisms. The activities of key enzymes in glycerol degradation pathways were determined for organisms that use different strategies for its utilization. It was shown that Halanaerobium saccharolyticum and Anoxynatronum sibiricum employ fundamentally different mechanisms for the glycerol degradation in the absence of an external electron acceptor. In the former, the disposal of the generated reducing equivalents occurs via the classical propanediol pathway, while in the latter, it involves the formate dehydrogenase complex. The example of Alkaliphilus peptidifermentans serves to demonstrate the role of protein hydrolysate components as acceptors, so that bacteria unable to ferment glycerol can nevertheless assimilate it.